The present invention relates to a metal ring gear having roll formed teeth and to a method of making such a gear.
Metal sprockets and gears are manufactured by many different methods. Thus, metal gears may be cast and appropriately finished, cut from metal blanks utilizing gear hobbing equipment or the like, drawn from plate material in a die, or cold formed using various rolling tools and methods. Cold rolled gears are known to possess certain advantages over cut gears, such as eliminating waste chip material, providing uniform work hardening of the teeth during formation, and providing high tolerance and a good surface finish.
U.S. Pat. No. 3,745,851 shows a toothed gear or sprocket formed from a flat circular disc on which a peripheral tooth pattern is formed by rotating the disc between a pair of diametrically opposed rotating forming tools. The disc and the two oppositely disposed forming tools all rotate on parallel axes. Although the formed teeth are widened somewhat from the thin body of the disc in forming, the amount of material available and thus the axial width of the teeth are limited. U.S. Pat. No. 4,132,098 shows a similar process for cold rolling gears utilizing a single gear forming roll which rotates with the blank relative to its peripheral edge to cold form the tooth pattern therein. To provide a wide tooth face, suitable wide gear blanks are utilized.
In some applications, it is desirable to provide a gear having a relatively thin disc-shaped body and a substantially wider axial tooth pattern on its peripheral edge. For example, a starting ring gear for an automotive engine desirably has a substantially flat thin disc-shaped body, just sufficient to provide the necessary strength and rigidity while preserving a light weight, and having a peripheral tooth pattern in which the teeth have a substantially wider axial dimension. Such gears are typically formed from castings or forgings made with an axially widened peripheral edge into which the tooth pattern is subsequently cut with conventional gear cutting equipment. Such gears, however, bear the disadvantages which are attendent cut gears generally, as discussed above.
A ring gear might also be made by drawing a circular metal disc into a cup shape to form an axially extending cylindrical peripheral flange and forming a tooth pattern on the outer flange surface. The tooth pattern might be cut or cold formed by drawing, the latter in accordance with a method generally shown in U.S. Pat. 3,796,085. Teeth may also be cold formed in the axially extending outer cylindrical flange by the use of a known cold rolling method in which a gear forming roller or rollers are advanced incrementally in an axial direction across the surface of the flange sequentially with circumferential indexing around the flange. However, utilizing a gear blank in which the cylindrical outer flange is drawn from the blank or comprises a separate cylindrical ring welded to the outer edge of a flat disc presents inherent strength limitations and/or involves costly and time consuming fabricating steps. Also, the use of prior art roll forming methods requires the ID of the cylindrical flange to be supported on a grooved or splined mandrel of the same diameter. In addition, the depth and, therefore, the strength of the tooth pattern formed in the cylindrical outer ring is limited.
It would be desirable, therefore, to be able to provide a cold rolled gear possessing all the benefits of a gear formed by known cold forming methods, yet having an overall strength and tooth pattern comparable to conventional cut gears.